Use of paint rock in foundry practice



United States Patent ABSTRACT or THE DISCLOSURE A substantial saving in the cost of a conventional foundry sand composition is realized through substituting aint r r (a) a iig n ifiga nt p rtion of the conventional bentonite content and (b) all of the conven- 'tlohal content of ir of.

This invention relates to the foundry casting art, and is concerned with the provision of a foundry sand com- 2 position enjoying the favorable properties of conventional foundry sand compositions while representing a substantial reduction in the over-all cost thereof.

In modern foundry practice many types of materials, such as a variety of clays, various types of sand and a host of various binders and additives such as molasses, pitch, bentonite, lignins, iron oxides, wood flour, core oil and others, are used in various combinations to produce the optimum molds and cores for various types of castmgs.

Iron oxides heretofore have been used for both core sand mix and for molding sand. The normal iron oxide serves a number of purposes, such as to provide chilling, to reduce penetration and to prevent veining. In molding sands it acts as inert fines, as a less hazardous substitute for silica fiour or silt. It also has a beneficial effect where kaolin isused since it makes it easier to revive the used molding sand (for re-use) after being heated to around 1000" 'F.

Most molding sands contain some bentonite, either of the Southern or non-swelling type or the Western or swelling variety, to give the mix plasticity and high dry strength, improve green strength, resist scabbing when subject to heat shock, and provide better quality pattern reproduction.

Bentonite, otherwise known as colloidal clay (The Condensed Chemical Dictionary, 7th ed.) contains appreciable amounts of the mineral montmorillonite. It is composed principally of aluminum silicates, usually with some magnesium and iron. Its outstanding characteristics of extreme fineness and plasticity account for its use in oil well drilling muds; as bonding agent in various relations; in pelletizing fine ores; in foundry sands; and otherdiverserelations.

In the search for more effective and cheaper foundry sand binders, I have discovered a material that combines the properties of two widely used binders, namely, iron oxide and bentonite, resulting in a much improved product over the use of the other two and one which can be supplied to the industry at a considerably lower cost. This new material is known throughout the iron industry as Paint Rock. It is best known for its occurrence in the Biwabik Iron Formation on the Mesabi Range of Minnesota where it has been formed as a secondary product through the oxidation of the original slatey members. It does not have a definite mineralogical composiice tion but is made up mainly of a mixture of hematite, kaolinite and quartz. A typical chemical composition, in parts by weight, of the paint rock on a dry basis as mined at the WhitesideMine of the Snyder Mining Co. at Buhl, Minn., is as follows:

Iron, 51.40; Phos., .112; Silica, 8.00; Mang., 0.94; Alum, 7.13; Loss by ignition, 10.0; Moisture, 19.7.

Paint rock occurs as a fairly soft rock, red to orange in color, and is very sticky when wet. The minerals themselves are extremely fine in size since they are derived from the alteration of a slate, by definition a fine grained rock derived from the solidification of a mud. The extreme stickiness of the paint rock is due not only to the fineness of the mineral constituents but particularly to the presence of kaolinite which is commonly known as clay. Paint rock has been described in detail in the following reference:

Gruner, J. W.; Mineralogy and Geology of the Mesabi 0 Range: Minnesota Geological Survey, St. Paul, Minnesota, 1946.

I have found that paint rock, to be used successfully in core sand, must be carefully prepared in the following manner:

I have found that the first stage in the preparation of the paint rock after mining and coarse crushing to about 4 inches, is the drying operation that must be done carefully. It may be noted that the material is high in moisture (19.7%) and also has a high loss on ignition (10%). This loss on ignition is almost entirely made up of the chemically combined water, either as water of crystallization or as the hydroxyl molecule present in the hydrous minerals in the paint rock, the principal one of which is kaolinite. I have learned that it is desirable to dry this paint rock so that the amount of moisture is reduced to below 6% but in so doing it is imperative that none of the paint rock be subjected to a temperature sufiiciently high that the combined water will be driven off. If any part of the ore is heated above about 350 F. there is a chance that the combined water may be driven off and the nature of the mineral changed. If such dehydrated mineral is brought again into contact with water, it will not retake water but will remain inert plastically and will be useless as a binder.

Mention has been made that the moisture be reduced below 6%. The absolute figure will vary with the type of paint rock used. It is, of course, necessary that the ground material be not sticky so that it will not hang up in bins or conveyor pipes. Overdrying beyond this simply means additional expense. However, drying of some types of paint rock down to 1 or 2% moisture may be necessary to secure efiicient fine grinding and the avoidance of caking in grinding mills, pipes and storage bins. As a usual thing I prefer drying in two stages-the first step to be carried out in a rotary dryer with careful temperature controls. The second step combines the fine grinding down to 200 mesh stage with the final drying operation, effected in a hot air-swept ball or roller mill. The use of paint rock as a substitute for the normal iron oxide has been found to be most eflFective not only in the laboratory but in a commercial foundry making large castings.

EXAMPLE #1 The following are the results from using the so-prepared paint rock material as a direct replacement for a commercially available iron oxide in the production of steel castings. The core sand mix employed was the following:

Lbs.

Silica sand 800 Paint rock 2.7 Sodium silicate 40 Paint rock Iron oxide Gassed (only) 30-30-19 28-37-14 P.s.l Avg. 29 Avg. 29 Baked 450 298-219-328 276-288-257 After gassing, p Avg. 281 Avg. 273

The above comparative data show that the prepared paint rock material did not reduce tensile properties, and actually, in baked condition, produced higher strength than the conventional mix. A 635 pound valve body was poured using the paint rock core. Surface finish and casting appearance were equal to, or better than, those obtained with conventional sand mix.

It seems clear that these properties, as regards use in core sand mixes may be explained on the basis of the following facts:

Paint rock is a superior material to the standard iron oxides now used in foundry sand compositions because it is finer grained and therefore is more easily dispersed. It has the additional advantage that it is also very plastic and an excellent binder and, therefore, effectively replaces a portion of the bentonite commonly used in such compositions.

Although the invention has been described as an example with reference to carbon dioxide contact of sodium silicate treated sand, it should be understood that other acid reacting substances, singly or in combination,

may be employed. Similarly the process described herein is by no means limited to silica sand, but may be used in conjunction with other granular materials such as chamotte, chromite, cement, etc., which may be formed into shaped articles.

4 I claim: 1. A core sand mixture consisting essentially of the following components in the following relative amounts:

Lbs.

Silica sand 800 Paint rock comprising hematite, kaolinite and quartz 2.7

Sodium silicate 40 2. Process of forming a core for use in the production of a metal casting which comprises preparing naturally occurring paint rock, comprising hematite, kaolinite and quartz, by drying the same at a temperature below that necessary for driving off either water of crystallization or water of hydration, to a moisture content of less than 6% by weight and subdividing the dried paint rock to substantially all minus 200 mesh (Tyler standard screen), mixing the so-prepared paint rock with silica sand and sodium slicate in the relative amounts, expressed in parts by weight:

Lbs. Silica sand 800 Prepared paint rock 2.7 Sodium silicate 40 forming the mixture into a core of desired shape, and treating the formed core with carbon dioxide gas to set the sodium silicate.

References Cited UNITED STATES PATENTS 2,256,047 9/1941 Dietert 10638.27 XR 2,504,133 4/1950 Kerlin 10638.9 2,883,723 4/1959 Moore et a]. 10638.3 XR 2,975,494 3/1961 Cooper 10638.35 XR JULIUS EROME, Primary Examiner L. B. HAYES, Assistant Examiner US. Cl. X.R. 

